Sunglasses are popular as fashion items, but are also used to protect the wearer's eyes from harmful effects of sunlight, such as cataracts, macular degeneration, and photokeratitis. Traditional materials used to make lenses for sunglasses include plastic (polycarbonate and CR-39 resins) and crown glass. Such lenses are generally tinted to reduce the transmittance of the lens, or include a polarized layer that reduces light intensity by about 50% and can reduce the glare of reflected light. Typically, the lenses of sunglasses are invariant, in that such lenses exhibit only one fixed color state, and their transparency is not variable (other than in regard to the polarization angle of the light relative to the lens polarization axes—if polarized lenses are used). During outdoor activities such as motorcycling and skiing, sunlight conditions can vary considerably, and invariant conventional lenses cannot adjust to such varying conditions of brightness in the ambient light.
Photochromic lenses were developed to address this issue. Photochromic lenses incorporate light sensitive molecules into the lens (or into a film applied to the lens). Such light sensitive molecules cause the lenses to become less transparent when exposed to ultra-violet (UV) radiation. Once the UV component of the ambient light is substantially reduced (for example, by walking indoors), the lenses gradually return to their clear state. Photochromic glass lenses generally incorporate silver halides into the lens, while polymer photochromic lenses employ organic molecules, such as oxazines and napthopyrans.
Typically, photochromic lenses darken substantially in response to UV light in less than one minute, and then continue to darken very slightly over the next fifteen minutes. As soon as exposure to the UV light ceases, the lenses begin to clear, becoming noticeably less tinted within two minutes, and are generally transparent within five minutes. However, it normally takes more than fifteen minutes for the lenses to become completely transparent.
In addition to the relatively slow response time of these passive photochromic lenses, such lenses exhibit temperature dependency, which prevents such lenses from achieving the darker tints in hot weather that they do when exposed to milder weather. In contrast, photochromic lenses achieve deep tints when exposed to cold weather conditions. Thus, photochromic lenses are more suitable for snow skiers than beachgoers. The temperature dependency also increases the time required for tinted lenses to return to their transparent state after exposure of the lenses to UV radiation has been terminated.
Yet another limiting factor for photochromic lenses is that they respond only to UV radiation, and not visible light. Because most vehicle windows act as a UV filter, photochromic lenses will not be exposed to much UV radiation when worn in a car, and thus, will not achieve the deeper tints for blocking light desired for sunglasses in bright environments.
In contrast, smart color change materials (as opposed to the passive photochromic materials discussed above) are characterized by their ability to vary their transparency (i.e., their transmittance values) upon application of an electric potential across the materials. Smart color change materials include suspended particles, liquid crystals, and electrochromics. Suspended particle devices (SPD) and liquid crystal devices (LCD) are capable of rapid switching, and do not suffer from the UV and temperature dependencies of passive photochromic materials. However, SPDs and LCDs require high voltages be applied to control light transmittance. Further, they are characterized by relatively high production costs, complex manufacturing requirements, lack of memory function, and limited color availability that has prevented them from being used in smart sunglasses and goggles.
Electrochromic (EC) materials can change their color when an electrical potential is applied, due to electrochemical oxidation and reduction reactions occurring within the materials. However, EC materials based on inorganic transition metal oxides (such as WO3) have relatively slow response times (on the order of tens of seconds), and relatively high processing costs.
EC polymers are more promising materials for use in sunglasses. EC polymer based devices (ECDs) exhibit several desirable characteristics. They require power only during switching state; their operating voltages and energy consumption are low; they have rapid response times; they exhibit an open circuit memory function; they exhibit great repeatability; they offer rich color choices; and they are relatively easy to manufacture. Based on these characteristics, it clearly would be desirable to provide smart eyewear incorporating EC polymers having these advantages.